Re: I believe there could be two masked men.
Well, I agree with the statement that there might be two masked men.
One with long hair, and then Obito - And somewhere along the road, the "tourch" was passed on to Obito
But the one fighting Minato is Obito. And the age isn't a problem.
In the flashback where we see Kakashi, Gai and the others getting ready for the battle against Kurama and the masked man (although they aren't allowed to participate), they had grown since Obitos "death". And there is no problem for them to be the same height as an adult.
This can be seen here:
I simply think that Obito have
1. Learned alot of stuff from a tutor during the time (Madara, or Zetsu with madaras knowledge perhaps)
2. Gotten some knowladge/memories from Madara, through dna or chackra
In any way, we've seen that some of his body parts have been replaced with Zetsu parts (His arm "melting" away), or in some other way been altered to have a similar ability